This doesn't show up until you apply the HDFS-2492 patch, but the attempt to make the StaticMapping topology clever by deciding if it is single rack or multi rack based on its rack->node mapping breaks the HDFS TestBlocksWithNotEnoughRacks test. Why? Because the racks go in after the switch topology is cached by the BlockManager, which assumes the system is always single-switch.
Fix: default to assuming multi-switch; remove the intelligence, add a setter for anyone who really wants to simulate single-switch racks.
Test: verify that a newly created simple mapping is multi switch